The present invention pertains to a lubricant containing particles, especially a liquid lubricant such as an oil or a lubricating grease containing particles of two different particles sizes, wherein the difference between the sizes of the particles is in the range of 1:100 or higher.
The lubricant is intended especially for use in ball-and-socket joints which are used, e.g., in steering linkages of motor vehicles. Use of the lubricant in other bearings or for other lubrication purposes is also possible.
Lubricants which contain particles with two different particle sizes have been known from JP-A 63 172 795, U.S. Pat. No. 4,888,122 or DE 198 39 296 A1. The difference in the size of the particles may be in the range of 1:100 or more. The purpose of the lubricant according to the first document mentioned is good heat resistance and lubrication properties at high temperatures as well as high stress. The purpose of the lubricant according to the second document mentioned is to coat porous bearing surfaces of an internal combustion engine with the particles in order to smooth the surface. The purpose of the third document mentioned is to damp rattling vibrations especially in self-locking transmissions of hinge mounts on vehicle seats. The prior-art lubricants are not intended specifically for ball-and-socket joints and do not solve the problems described below which occur in ball-and-socket joints.
If a ball-and-socket joint that is at rest is to be moved (pivoted), a type of breakaway torque, i.e., an increased torque must first be applied in order to set in motion a ball socket of a ball-and-socket joint in relation to a ball of the ball-and-socket joint. When the ball socket is moving in relation to the ball, the torque decreases, doing so usually abruptly, to considerably less than half of the breakaway torque. The beginning of the pivoting of the ball-and-socket joint from the state of rest is accompanied by a jerk, which can be felt in some cases and is sometimes also audible as a clicking. This jerk and the breakaway torque at the beginning of the pivoting of the ball-and-socket joint from the state of rest is explained by the ball of the ball-and-socket joint displacing grease or another lubricant from a point of the ball-and-socket joint with the ball-and-socket joint at rest. The lubricating film thickness decreases in this point to a fraction of the lubricating film thickness present in the moving ball-and-socket joint, and the lubricating film thickness drops to zero in the extreme case. The reduction in the lubricating film thickness is time-dependent. The friction of the ball-and-socket joint increases, e.g., threefold to fourfold with decreasing lubricating film thickness. This high friction must be overcome at the beginning of the pivoting of the ball-and-socket joint. When the ball is moving in the ball socket, lubricant is distributed over the surface of the ball, as a result of which the friction of the ball-and-socket joint decreases.
To manufacture a ball-and-socket joint, the ball socket is first manufactured as a hemisphere with a hollow cylindrical edge attached in one piece and smoothly to the hollow hemisphere in order to insert the ball into the ball socket. After the ball has been inserted into the ball socket, the hollow cylindrical edge is deformed inwardly, so that the ball socket surrounds the ball over more than a hemispherical surface and thus holds it by extending behind it in a positive-locking manner. A friction-reducing bearing shell made of plastic, e.g., polyacetate (POM), is often placed into the ball socket. A lubricant, mostly a lubricating grease, is applied to the ball and/or the ball socket or the bearing shell before the ball is introduced into the ball socket. After the ball has been placed into the ball socket and the hollow cylindrical edge of the ball socket has been deformed inwardly to enclose the ball, the ball-and-socket joint is heated. The purpose of this is to adapt the bearing shell to the shape of the ball. A gap between the ball and the ball socket, which is necessary for the pivotability of the ball-and-socket joint, becomes established due to the fact that the hollow cylindrical edge of the ball socket will again expand after the deformation in the inward direction.
The problem arises that a gap becomes established between the ball and the ball socket during the deformation. However, this spontaneous establishment of the gap between the ball and the ball socket is inaccurate and may adversely affect the parameters of the ball-and-socket joint.
The basic object of the present invention is to provide a lubricant which makes possible the accurate establishment of the gap between the ball and the ball socket during the manufacture of a ball-and-socket joint.
According to the invention, a lubricant is provided containing particles of two different particles sizes. The difference between the sizes of the particles is in the range of 1:100 or higher. The coarser particles have a lower dissolution temperature in the lubricant than do the finer particles.
The dissolution temperature is the temperature beginning from which the particles become dissolved in the lubricant or, what actually happens to the particles, the particles disappear when the particles are viewed under the microscope and, what is essential, they no longer reappear after the lubricant is cooled to below the dissolution temperature. After heating and optionally holding the lubricant at the dissolution temperature of the coarser particles and subsequent cooling, the coarser particles are no longer present as such. It is thus possible to dissolve the coarser particles by heating the lubricant according to the present invention to or above the dissolution temperature of the coarser particles but not to the dissolution temperature of the finer particles and by holding the lubricant at this temperature. The lubricant according to the present invention has the following advantage during the manufacture of a ball-and-socket joint: The coarser particles of the lubricant keep the ball socket at a spaced location from the ball during the deformation of the hollow cylindrical ball socket to enclose the ball of the ball-and-socket joint seated in the ball socket. The gap present between the ball socket and the ball after the deformation of the ball socket can be set very accurately by selecting the diameter of the coarser particles. The ball-and-socket joint is subsequently heated to the dissolution temperature of the coarser particles and held temporarily at this temperature, as a result of which the coarser particles dissolve, whereas the finer particles with the higher dissolution temperature continue to be present. Since the coarser particles are not formed again after the cooling of the lubricant, only the finer particles will remain in the lubricant and form the spacers, which maintain the distance (gap) between the ball and the ball socket even when the ball-and-socket joint is at rest, to prevent the ball from lying on the ball socket and thus avoid the breakaway torque of the ball-and-socket joint.
Coarser particles and a bearing shell whose dissolution temperatures or softening points are approximately equal and are lower than the dissolution temperature of the finer particles are preferably selected. In the case of polyacetate (POM) as the material of the bearing shell, which softens at approximately 100xc2x0 C. and begins to melt beginning from approx. 120xc2x0 C., the coarser particles selected for the lubricant according to the present invention have a dissolution temperature of approx. 80-100xc2x0 C. and the finer particles selected have a dissolution temperature of approx. 120xc2x0 C. The coarser particles are made of, e.g., polyethylene and the finer particles from polyamide with a higher dissolution temperature in the lubricant.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.